In a photo provided by Showtime, Claressa Shields follows through on a left to Tori Nelson during a boxing bout Friday night, Jan. 12, 2018, in Verona, N.Y. Shields scored a unanimous 10-round decision to retain her women's WBC and IBF super middleweight world titles. (Stephanie Trapp/Showtime via AP)

In a photo provided by Showtime, Claressa Shields is interviewed after he win over Tori Nelson during a boxing bout Friday night, Jan. 12, 2018, in Verona, N.Y. Shields scored a unanimous 10-round decision to retain her women's WBC and IBF super middleweight world titles. (Stephanie Trapp/Showtime via AP)

Each of the three judges scored the fight 100-90 for Shields (5-0, 2 KOs), of Flint, Michigan, who was nearly 20 years younger than her opponent. The 41-year-old Nelson (17-1-3, 1 KO), who won a middleweight world title in 2011 but had only fought twice since June 2015, lost for the first time in her career.

Nelson, of Ashburn, Virginia, took the fight to Shields at the outset, but as the bout wore on the young champion proved to be the more skilled boxer, able to avoid Nelson's big hooks and using her jab to keep her foe at bay.

Nelson landed a sharp body shot in the seventh round, putting Shields on the ropes and landing several more punches. But the 22-year-old Shields, who likes to boast she's the greatest of all-time, dodged most of them and landed a right hand of her own that stunned Nelson and sent her backward.

WBO and WBC middleweight champion Christina Hammer (21-0, 9 KOs) watched the fight from ringside. Shields said after her victory Friday night, which was televised live on Showtime, that she could easily go down in weight to face Hammer, who is from Kazakhstan.

"I think she's pretty scared, to be honest. She's more worried about me than I am her," Shields said after her win. "I can't wait to fight her and show her that she isn't (better than me)."

Shields pummeled Nikki Adler last August to win the two super middleweight titles. That lopsided fight, Adler's first loss in 17 bouts, was stopped in the fifth round. Shields' combinations left Adler defenseless in a matchup of previously unbeaten boxers.

Shields displayed her dominance in the punching totals on Friday night, connecting on 225 of 632 overall and 186 of 425 power punches to Nelson's 81 of 400 and 70 of 348.

After the fight, Shields donated her gloves to the International Boxing Hall of Fame in nearby Canastota, New York.

Shields became the first woman to headline a fight card on premium cable in March and again last August on Showtime. She's the first American, male or female, to win a boxing gold medal since Andre Ward at the 2004 Athens Olympics.

The only other U.S. boxer to win two Olympic gold medals is Oliver Kirk, who took the bantamweight and featherweight titles at the 1904 Olympics in St. Louis.